With the development of radio multimedia services, people have increasing requirements for high data rate and user experience, so higher requirements are made on system capacity and coverage of the traditional cellular network. On the other hand, the popularity of social network, near-field data sharing, local advertising and other applications increases demands of people on knowing persons or things of interest nearby and communicating with them. The existing cellular network based on a cell has obvious limitations on supporting high data rate and neighboring services; in this background, a Device-to-Device (D2D) technology representing a new development trend of communication technology in future emerges at the right moment. The application of the D2D technology can lighten load of the cellular network, reduce the power consumption of battery of a User Equipment (UE), improve data rate, and improve the robustness of network infrastructure, thereby meeting requirements of the high data rate services and neighboring services well.
The D2D technology can work on an authorized frequency hand or an unauthorized frequency band, allowing multiple pieces of UE supporting a D2D function (namely a D2D UE) to perform direct discovery/direct communication in the presence of network infrastructure or in the absence of network infrastructure. There are mainly three application scenarios of the D2D technology:
(1) UE1 and UE2 perform data interaction under the coverage of the cellular network, and user plane data does not go through the network infrastructure, as mode 1 in FIG. 1;
(2) UE in an area with poor/no coverage performs relay transmission; as mode 2 in FIG. 2, UE4 with poor signal quality is allowed to communicate with the network through UE3 with network coverage nearby, thereby helping operators to extend coverage and improve capacity; and
(3) when the cellular network cannot work normally because of earthquake or emergency, devices are allowed to communicate directly; as mode 3 in FIG. 1; user planes and user planes among UE5, UE6 and UE7 perform single-hop or multi-hop data communication not through the network infrastructure.
The D2D technology usually includes D2D discovery technology and D2D communication technology:
(1) the D2D discovery technology is used for judging/determining two or more pieces of D2D UE are adjacent to each other (e.g. in a range that they can perform direct D2D communication), or used for judging/determining that a first UE is adjacent to a second UE;
(2) the D2D communication technology is that direct communication can be performed on part or all of communication data among the D2D UE not through the network infrastructure.
In the application scenarios shown in FIG. 2 and FIG. 3, the D2D UE can serve as a relay node, enabling a remote D2D UE at the edge or out of coverage of the cellular network to perform cellular communication with the network through the relay node, and enabling the D2D UE to perform D2D communication through the relay node. It is impossible that the D2D UE moves at the edge of cell coverage, then the D2D UE needs to switch its access node to access the network; for example, after moving to the cell coverage, the D2D UE relaying through a relay UE can access the network through a base station; after moving out of the cell coverage, the UE accessing the network through the base station in the cell coverage can access the network only through the relay UE. However, the prior art does not provide a method for the D2D UE to change an access node, thereby influencing communication service continuity of the UE at the edge of cell coverage.